Monday, November 25, 2013

Reading List: Starship Century

“Is this the century when we begin to build starships?”
So begins the book, produced in conjunction with the
Starship Century
Symposium held in May of 2013 at the University of California San
Diego. Now, in a sense, we built and launched starships in the
last century. Indeed, at this writing, eight objects
launched from Earth are on
interstellar trajectories.
These are the two
Pioneer
spacecraft, the two
Voyagers, the
New Horizons
Pluto flyby spacecraft, and its inert upper stage and two
spin-down masses. But these objects are not aimed at any particular
stars; they're simply flying outward from the solar system following whatever
trajectory they were on when they completed their missions, and even
if they were aimed at the nearest stars, it would take them tens of
thousands of years to get there, by which time their
radioactive power
sources would be long exhausted and they would be inert space junk.

As long as they are built and launched by beings like humans (all bets
are off should we pass the baton to immortal machines), starships or
interstellar probes will probably need to complete their mission
within the time scale of a human lifetime to be interesting. One can
imagine multi-generation colony ships (and they are discussed here),
but such ships are unlikely to be launched without confidence the
destination is habitable, which can only be obtained by direct
investigation by robotic probes launched previously. The closest star
is around 4.3 light years from Earth. This is a daunting distance.
To cross it in a human-scale time (say, within the career of a
research scientist), you'd need to accelerate your probe to something
on the order of 1/10 the speed of light. At this speed, each kilogram
of the probe would have a kinetic energy of around 100 kilotons of
TNT. A colony ship with a dry mass of 1,000 tonnes would, travelling
at a tenth of the speed of light, have kinetic energy which, at
a cost of USD 0.10 per kilowatt-hour, would be worth USD 12.5
trillion, which is impressive even by U.S. budget deficit standards.
But you can't transmit energy to a spacecraft with 100% efficiency
(the power cord is a killer!), and so the cost of a realistic
mission might be ten times this.

Is it then, silly, to talk about starships? Well, not so fast. Ever
since the Enlightenment, the
GDP
per capita has been rising rapidly. When
I was a kid, millionaires were exotic creatures, while today people
who bought houses in coastal California in the 1970s are all
millionaires. Now it's billionaires who are the movers and shakers,
and some of them are using their wealth to try to reduce the cost
of access to space. (Yes, currency depreciation has accounted for a
substantial part of the millionaire to billionaire transition, but the
scope of what one can accomplish with a billion dollar grubstake today
is still much greater than with a million dollars fifty years ago.)
If this growth continues, might it not be possible that before this
century is out there will be trillionaires who, perhaps in a consortium,
have the ambition to expand the human presence to other stars?

This book collects contributions from those who have thought in
great detail about the challenges of travel to the stars, both in
nuts and bolts hardware and economic calculations and in science
fictional explorations of what it will mean for the individuals
involved and the societies which attempt that giant leap. There
are any number of “Aha!” moments here.
Freeman Dyson points out that the void between the stars is
not as empty as many imagine it to be, but filled with
Oort cloud
objects which may extend so far as to overlap the clouds of
neighbouring stars. Dyson imagines engineered organisms which
could render these bodies habitable to (perhaps engineered) humans,
which would expand toward the stars much like the Polynesians
in the Pacific: from island to island, with a population which would
dwarf both in numbers and productivity that of the inner system
rock where they originated.

We will not go to the stars with rockets like we use today. The most
rudimentary working of the numbers shows how absurd that would be.
And yet
nuclear thermal rockets, a technology developed and tested
in the 1960s and 1970s, are more than adequate to develop a solar
system wide economy which could support interstellar missions. Many
different approaches to building starships are explored here: some
defy the constraints of the
rocket equation
by keeping the power source in the solar system, as in
“sailships” driven by laser or microwave radiation.
A chapter explores “exotic propulsion”, beyond our
present understanding of physics, which might change the game.
(And before you dismiss such speculations, recall that according
to the consensus model of cosmology, around 95% of the universe is
made up of “dark matter” and “dark energy” whose
nature is entirely unknown. Might it be possible that a
vacuum propeller
could be discovered which works against these pervasive media just as
a submarine's propeller acts upon the ocean?)

Leavening the technical articles are science fiction stories exploring
the transition from a planetary species to the stars. Science fiction
provides the dreams which are then turned into equations and
eventually hardware, and it has a place at this table. Indeed, many
of the scientists who spoke at the conference and authored chapters
in this book also write science fiction. We are far from being able
to build starships or even interstellar probes but, being human, we're
always looking beyond the horizon and not just imagining what's
there but figuring out how we'll go and see it for ourselves. To date, humans haven't
even learned how to live in space: our space stations are about
camping in space, with extensive support from the Earth.
We have no idea what it takes to create a self-sustaining closed
ecosystem (consider that around 90% of the cells in your body are not
human but rather symbiotic microbes: wouldn't you just hate it to be
half way to Alpha Centauri and discover you'd left some single-celled
critter behind?). If somebody waved a magic wand and handed us a
propulsion module that could take us to the nearest stars within a
human lifetime, there are many things we'd still need to know in order
to expect to survive the journey and establish ourselves when we
arrived. And, humans being humans, we'd go anyway, regardless.
Gotta love this species!

This is an excellent survey of current thinking about interstellar
missions. If you're interested in this subject, be sure to view the
complete
video archive of the conference, which includes some
presentations which do not figure in this volume,
including the magnificent
galaxy garden.